The injection of Gross virus-induced C58NTD lymphoma cells into syngeneic rats leads to significant changes in the profile of the classical and alternative pathway components of the complement system. Total hemolytic complement activity as well as that of C1 and C3 are markedly reduced. Antigenic assays of serum properdin levels are also lowered within one week following the injection of the tumor cells. These findings are now being extended in studies of a more chronic system of chemical carcinogenesis in inbred rats to ascertain whether the appearance and growth of autochthonous tumors are also accompanied by serum complement alterations. A method has been devised to estimate antigen specific IgM and IgG immunoglobulins at the nanogram level. Experiments have been designed to assess the possibility that depletion of C3 influences the early immune response to cellular and soluble antigens. It is anticipated that antibody weight rather than activity measurements will hopefully clarify the putative role of C3 in early antibody synthesis. This antibody assay will also be applied to studies of the host's immune response during the early stages of autochthonous tumor development.